Motor driven exercise machines, such as treadmills, have become increasingly popular in recent years. With this popularity, the complexity of such devices has also increased. For example, many treadmills include speed and incline control. One effective way to employ speed control is to use electronically commutated motors. Such motors typically use one or more high power solid state switches to selectively connect a power source to the motor. This selective application, commonly referred to as "commutation," can accurately control the speed of the motor, and hence, the speed of the treadmill.
Of course, safety is also of concern to those who either use or manufacture and sell motorized exercise machines. Safety in this sense includes both the safety of the user as well as safety of the electrical and mechanical components that comprise the machine. For example, in treadmills using a solid state power switch to commutate power to the motor, it is important to account for the possibility that such a switch might fail--if the commutation switch fails in a short circuit state, full power is applied to the motor. Such a condition could result in overspeeding the treadmill as well as damaging the apparatus. Therefore, many motorized treadmills include shutdown means for removing power when such conditions are detected.
Several methods for accomplishing this safety shutdown function have been devised. For example, inline fuses can be installed that "blow" upon prolonged overcurrent conditions. Such an approach is employed in the treadmill safety module depicted in U.S. Pat. No. 5,571,062. This approach, while partially effective, has several drawbacks. First, if "slow blow" fuses are used, shutdown is delayed. This can result in potential risk to both the user and the components of the machine. For example, the motor may burn up or otherwise sustain damage. Second, if "fast blow" fuses are used, spurious undesirable shutdowns might occur due to transients. This latter problem is compounded by the fact that the user must then replace the blown fuse, and if the problem was more serious than a mere transient, the replacement fuse will also blow. Thus, it may take the failure of two fuses before the treadmill user can determine that the machine has experienced a serious failure.
Another method for detecting failures measures the voltage drop that develops naturally between the motor and the commutation switch. In particular, connecting the commutation switch between the motor and ground creates a voltage divider comprising the motor impedance and the short circuit switch impedance. The voltage developed at a point between the switch and the motor is monitored. If the switch fails in a short circuit state, that voltage will approach ground potential. One drawback of this latter approach, however, is that it is not as effective if used with low impedance motors that are available. Such differences in impedance can occur, for example, due to the use of different wire gauges. If a low impedance motor is used, the motor itself does not provide much inherent current limiting. Therefore, more current flows through the shorted switch causing a relatively higher voltage to develop across the switch. This relatively higher voltage may provide a false indication that the switch is operating correctly. As the motor speed increases the impedance of the motor also increases, and, eventually, the shutdown can occur. This delay is not optimal. Hence, the safety shutdown circuit must be designed to account for the impedance of the motor installed and a "generic" circuit is not likely to work as effectively.
There is a need, therefore, for a low cost, highly reliable, and motor impedance independent safety shutdown circuit for use in motorized exercise machines.